drm/amd/display: use the proper fb offset for DM

This fixes DMCU initialization in APU GPU passthrough.  The
DMCU needs the GPU physical address, not the CPU physical
address.  This ends up working out on bare metal because
we always use the physical address, but doesn't work in
passthrough because the addresses are different.
Reviewed-by: NHuang Rui <ray.huang@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c

@@ -1096,7 +1096,7 @@ static int dm_dmub_hw_init(struct amdgpu_device *adev)
 	/* Initialize hardware. */
 	memset(&hw_params, 0, sizeof(hw_params));
 	hw_params.fb_base = adev->gmc.fb_start;
-	hw_params.fb_offset = adev->gmc.aper_base;
+	hw_params.fb_offset = adev->vm_manager.vram_base_offset;
 
 	/* backdoor load firmware and trigger dmub running */
 	if (adev->firmware.load_type != AMDGPU_FW_LOAD_PSP)
@@ -1218,7 +1218,7 @@ static void mmhub_read_system_context(struct amdgpu_device *adev, struct dc_phy_
 	pa_config->system_aperture.agp_top = (uint64_t)agp_top << 24;
 
 	pa_config->system_aperture.fb_base = adev->gmc.fb_start;
-	pa_config->system_aperture.fb_offset = adev->gmc.aper_base;
+	pa_config->system_aperture.fb_offset = adev->vm_manager.vram_base_offset;
 	pa_config->system_aperture.fb_top = adev->gmc.fb_end;
 
 	pa_config->gart_config.page_table_start_addr = page_table_start.quad_part << 12;